18 research outputs found
Femtosecond Laser Filamentation for Atmospheric Sensing
Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation
Impulsive rotational Raman scattering of N2 by a remote "air laser" in femtosecond laser filament
We report on experimental realization of impulsive rotational Raman
scattering from neutral nitrogen molecules in a femtosecond laser filament
using an intense self-induced white-light seeding "air laser" generated during
the filamentation of an 800 nm Ti: Sapphire laser in nitrogen gas. The
impulsive rotational Raman fingerprint signals are observed with a maximum
conversion efficiency of ~0.8%. Our observation provides a promising way of
remote identification and location of chemical species in atmosphere by
rotational Raman scattering of molecules.Comment: 4 pages, 4 figure
Coupled air lasing gain and Mie scattering loss: aerosol effect in filament-induced plasma spectroscopy
Femtosecond laser filament-induced plasma spectroscopy (FIPS) demonstrates
great potentials in the remote sensing for identifying atmospheric pollutant
molecules. Due to the widespread aerosols in atmosphere, the remote detection
based on FIPS would be affected from both the excitation and the propagation of
fingerprint fluorescence, which still remain elusive. Here the physical model
of filament-induced aerosol fluorescence is established to reveal the combined
effect of Mie scattering and amplification spontaneous emission, which is then
proved by the experimental results, the dependence of the backward fluorescence
on the interaction length between filament and aerosols. These findings provide
an insight into the complicated aerosol effect in the overall physical process
of FIPS including propagation, excitation and emission, paving the way to its
practical application in atmospheric remote sensing.Comment: 7 pages, 4 figure
Acoustic diagnostics of femtosecond laser filamentation
The promising application of femtosecond laser filamentation in atmospheric
remote sensing brings imperative demand for diagnosing the spatiotemporal
dynamics of filamentation. Acoustic emission (AE) during filamentation opens a
door to give the insight into the dynamic evolution of filaments in air. In
particular, the frequency features of the acoustic emission provide relevant
information on the conversion of laser energy to acoustic energy. Here, the
acoustic emission of femtosecond laser filament manipulated by energy and the
focal lengths was measured quantitatively by a broadband microphone, and the
acoustic parameters were compared and analyzed. Our results showed that the
acoustic power presents a squared dependence on the laser energy and the
bandwidth of the acoustic spectrum showed a significant positive correlation
with laser energy deposition. It was found that the spectrum of the acoustic
pulse emitted from the middle of the filament has a larger bandwidth compared
to those emitted from the ends of the filament and the spectrum of the acoustic
pulse is also an indicator of the filament intensity distribution. These
findings are helpful for studying the plasma filament properties and complex
dynamic processes through acoustic parameters and allow the optimization of
remote applications.Comment: 8 pages,5 figure
Second harmonic generation in a centrosymmetric gas medium with spatiotemporally focused intense femtosecond laser pulses
We demonstrate unexpectedly strong second harmonic generation (SHG) in Argon
gas by use of spatiotemporally focused (SF) femtosecond laser pulses. The
resulting SHG by the SF scheme at a 75 cm distance shows a significantly
enhanced efficiency than that achieved with conventional focusing scheme, which
offers a new promising possibility for standoff applications. Our theoretical
calculations reasonably reproduce the experimental observations, which indicate
that the observed SHG mainly originates from the gradient of nonuniform plasma
dynamically controlled by the SF laser field.Comment: 14 pages, 4 figure
Self-induced white-light seeding laser in a femtosecond laser filament
We report, for what we believe to be the first time, on the generation of
remote self-seeding laser amplification by using only one 800 nm Ti:Sapphire
femtosecond laser pulse. The laser pulse (~ 40 fs) is first used to generate a
filament either in pure nitrogen or in ambient air in which population
inversion between ground and excited states of nitrogen molecular ions is
realized. Self-induced white light inside the filament is then serving as the
seed to be amplified. The self-induced narrow-band laser at 428 nm has a pulse
duration of ~2.6 ps with perfect linear polarization property. This finding
opens new possibilities for remote detection in the atmosphere.Comment: 18 pages, 5 figure
Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses
We experimentally demonstrate ultrafast dynamic of generation of a strong
337-nm nitrogen laser by injecting an external seed pulse into a femtosecond
laser filament pumped by a circularly polarized laser pulse. In the pump-probe
scheme, it is revealed that the population inversion between the excited and
ground states of N2 for the free-space 337-nm laser is firstly built up on the
timescale of several picoseconds, followed by a relatively slow decay on the
timescale of tens of picoseconds, depending on the nitrogen gas pressure. By
measuring the intensities of 337-nm signal from nitrogen gas mixed with
different concentrations of oxygen gas, it is also found that oxygen molecules
have a significant quenching effect on the nitrogen laser signal. Our
experimental observations agree with the picture of electron-impact excitation.Comment: 9 pages, 5 figure
Femtosecond Filaments for Standoff Detection of Explosives
In this report, we present our results from various studies to qualitatively discriminate the common military explosives viz. RDX, TNT and HMX in their pure form at a distance of ~6.5 m in standoff mode using femtosecond (fs) filament induced breakdown spectroscopy technique (fs FIBS) together with principal component analysis. A ~30 cm length fs filament obtained by a two-lens configuration was used to interrogate those energetic molecules in the form of pressed pellets (150 mg each). The plasma emissions were collected by a Schmidt-Cassegrain telescope (SCT) from a distance of ~8 m away from the investigation zone. Additionally, a few significant results obtained from the LIBS-based investigations of nitroimidazoles with respect to the standoff distance (~2 m) are discussed. Furthermore, we have also summarised a few important results from our recent investigations of bulk energetic materials in various configurations (including those with fs filaments). The results obtained from various fs FIBS configurations corroborate that the filament generation and its properties, the size and f-number of collection optics influence signal strength in the FIBS technique. These results project the fs FIBS technique as a potential technique for investigations aimed at hazardous materials and harsh environments in the standoff mode